A prior generating system has been developed by the assignee of the instant application which includes a main generator, an exciter and a permanent magnet generator, (PMG), all of which are driven by a source of motive power and a constant speed drive (CSD). The frequency of the output power from the main generator is maintained within closely prescribed limits by means of a frequency control implemented by a microprocessor. The microprocessor controls a servo valve in the CSD, and hence the output speed of the CSD, depending upon the frequency of the waveform developed by the PMG. In such a frequency control, a divider/counter counts the number of pulses developed by a system clock between particular points in the wave form from the PMG to derive an indication of the period, and hence the frequency, of the waveform. Once the frequency indication is derived, an interrupt is coupled to the microprocessor which is processed by an interrupt handler to halt main program execution so that the data representing the frequency of the PMG output can be sensed by the microprocessor. Following the interrupt, main program execution resumes within the microprocessor so that it can accomplish the required control functions.
While the above system has been found satisfactory to control output frequency of a generator system such as that described above, the small number of interrupt inputs and handlers provided by the microprocessor has limited the ability of the microprocessor to control other functions in the generating system. When it is desirable or necessary to control functions in the system requiring interrupt inputs and handlers beyond that which can be accomodated by the microprocessor, an additional chip or chips must be provided to accomplish the interrupt handling function. Moreover, the software for handling an interrupt can become quite complex requiring additional programming above and beyond that required to implement the control functions.
The use of an interrupt procedure for data input also reduces the length of time remaining in each control cycle to accomplish control functions. In a real time based system, such as that described above, it would be desirable to reduce as much as possible the number of interrupts to be handled in a control cycle to in turn provide more time for other functions.
Other systems which rely upon an interrupt procedure to accomplish data input are disclosed in: Miyakawa U.S. Pat. No 4,404,648; Takahashi U.S. Pat. No. 4,398,260; Skarvada U.S. Pat. No. 4,270,176; and Goodson U.S. Pat. No. 4,291,878.